1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method of transmitting channel state information in a wireless communication system.
2. Discussion of the Related Art
Generally, a coordinated multi-point (CoMP) system (hereinafter abbreviated CoMP system) is the system to improve throughput of a user located on a cell boundary by applying enhanced MIMO transmission in a multi-cell environment. If the CoMP system is applied, it is able to reduce inter-cell interference in the multi-call environment. If the CoMP system is used, a user equipment can be supported with data from multi-cell base stations jointly.
And, each base station is able to enhance system performance by supporting at least one or more user equipments UE 1, UE 2, . . . , UE K simultaneously using the same radio frequency resource. Moreover, a base station is able to perform space division multiple access (SDMA) based on channel state information (CSI) between the base station and a user equipment.
The CoMP scheme can be categorized into a coordinated MIMO (Co0MIMO) type joint processing (JP) scheme through data sharing and a coordinated scheduling scheme/beamforming scheme (CS/CB).
FIG. 1 is a conceptional diagram for CoMP (coordinated multi-point) of an intra base station (intra eNB) and an inter base station (inter eNB) according to a related art.
Referring to FIG. 1, intra base stations 110 and 120 and an inter base station 130 exist in a multi-cell environment. In LTE (long term evolution) system, an intra eNB is constructed with several cells or sectors. Cells belonging to a base station, to which a specific user equipment belongs, lie in relation between the specific user equipment and the intra base stations 110 and 120. Namely, in case of sharing the same base station to which a user equipment belongs, the corresponding cells are the cells corresponding to the intra base stations 110 and 120. And, other cells belonging to other base stations becomes the cells corresponding to the inter base station 130. Thus, the cells, which are based on the same base station for a specific user equipment, exchange information (e.g., data, channel station information (CSI), etc.) with each other through x2 interfaces and the like. Yet, the cells, which are based on other base stations, can exchange inter-cell information with each other through a backhaul 140 and the like.
Referring to FIG. 1, a single cell MIMO user 150 in a single cell communicated with a single serving base station in one cell or sector. And, a multi cell MIMO user 160 located on a cell boundary is able to communicate with a plurality of serving base stations in multi cell or sector.
In the following description, spatial channel matrix usable for the present invention shall be schematically explained.
      H    ⁡          (              i        ,        k            )        =      [                                                      h                              1                ,                1                                      ⁡                          (                              i                ,                k                            )                                                                          h                              1                ,                2                                      ⁡                          (                              i                ,                k                            )                                                …                                                    h                              1                ,                Nt                                      ⁡                          (                              i                ,                k                            )                                                                                      h                              2                ,                1                                      ⁡                          (                              i                ,                k                            )                                                                          h                              2                ,                2                                      ⁡                          (                              i                ,                k                            )                                                …                                                    h                              2                ,                Nt                                      ⁡                          (                              i                ,                k                            )                                                            ⋮                          ⋮                          ⋱                          ⋮                                                                h                              Nr                ,                1                                      ⁡                          (                              i                ,                k                            )                                                                          h                              Nr                ,                2                                      ⁡                          (                              i                ,                k                            )                                                …                                                    h                              Nr                ,                Nt                                      ⁡                          (                              i                ,                k                            )                                            ]  
In this matrix, the H(i,k) is a spatial channel matrix, the Nr indicates the number of receiving antennas, the Nt indicates the number of transmitting antennas, the r indicates an index of a receiving antenna, the t indicates an index of a transmitting antenna, the i indicates an index of an OFDM or SC-FDMA symbol, and the k indicates an index of a subcarrier.
The hr,t(i,k) is an element of the channel matrix H(i,k) and means an rth channel state and a tth antenna on an ith symbol and a kth subcarrier.
Spatial channel covariance matrix usable for the present invention is schematically explained as follows. The spatial channel covariance matrix can be represented as a symbol R.R=E[Hi,kHi,kH]
In this matrix, the H indicates a spatial channel matrix and the R indicates a spatial channel covariance matrix. The E[ ] means an average, the i indicates a symbol index, and the k indicates a frequency index.
Singular value decomposition (SVD) is one of major methods for decomposing a rectangular matrix and is the scheme frequently used in the fields of signal processing and statistics. The SVD is generated from generalizing the spectrum theory of matrix for an arbitrary rectangular matrix. In case of using the spectrum theory, it is able to decompose an orthogonal square matrix into diagonal matrixes on the base of an eigen value. Assume that a matrix H is an m×m matrix consisting of elements of a set of real or complex numbers. In this case, the matrix H can be represented as multiplications of 3 matrixes shown in the following.Hm×m=Um×mΣm×nVn×nH 
In this case, the U and V indicate unitary matrixes, respectively. The Σ indicates m×n diagonal matrix including a singular value that is not negative. The singular value is represented as Σ=diag(σ1 . . . σr), σi=√{square root over (λi)}. Thus, the representation of the multiplication of three matrixes is called singular value decomposition. The singular value decomposition can handle more general matrixes rather than the eigen value decomposition capable of decomposing an orthogonal square matrix only. And, the singular value decomposition and the eigen value decomposition are related to each other.
When a matrix H is a positive definite Hermitian matrix, all eigen values of the H are non-negative real numbers. In this case, a singular value and vector of the H become equal to an eigen value and vector of the H, respectively.
Meanwhile, the eigen value decomposition (EVD) can be represented as follows.HHH=(UΣVH)(UΣVH)H=UΣΣTΣUH HHH=(UΣVH)H(UΣVH)H=VΣTΣV 
In this case, the eigen value can be set to λ1, . . . , λr.